1. Field of the Invention
This invention relates to improvements in charge pump circuits, and more particularly to improvements in the switching of current in a charge pump and more particularly to improvements in using field effect transistors as rectifiers in charge pump circuits.
2. Description of the Relevant Art
In many electronics applications, it is desirable to increase the voltage of a power source to a higher voltage. Prior art FIG. 1 shows a charge pump 10 which is a typical circuit for tripling the voltage of a power source. The charge pump 10 takes the supply voltage 12 and pumps it up to a pumped voltage Vp 14, which is between two and three times the supply voltage 12.
In the charge pump circuit 10, voltage source 12 is connected to the anode of diode 16. The cathode of diode 16 is connected to the anode of diode 18 and to the first plate of capacitor 22. The cathode of diode 18 is connected to the anode of diode 20 and the first plate of capacitor 24. A clock signal 26 is applied to the inputs of inverters 28 and 30. The output of inverter 28 is connected to the second plate of capacitor 22 and the output to inverter 30 is connected to the input to inverter 32. The output to inverter 32 is connected to the second plate of capacitor 24. The cathode of diode 20 is connected to filter capacitor 34, filter resistor 36, and the output node 14 at which pumped voltage Vp is presented.
In operation, capacitor 22 is charged to the voltage source level Vs through diode 16 when the clock signal 26 is high. When clock signal 26 goes low, the output of inverter 28 is driven to the voltage source level Vs. However, the voltage across capacitor 22 remains constant, pumping the voltage at the cathode of diode 16 to two times the supply voltage Vs minus the voltage drop across diode 16. At this time, the output to inverter 32 is low (ie, at ground). If the voltage across capacitor 24 is less than the pumped voltage at capacitor 22, the charge on capacitor 22 will flow through diode 18 on to capacitor 24 charging it to the pumped voltage. On the next clock cycle, the output of inverter 32 goes high, pumping the voltage at the cathode of diode 18 to three times Vs (less diode drops), since the voltage was previously two times the voltage Vs. For this reason, this charge pump circuit 10 is commonly referred to as a voltage tripler. This tripled voltage is filtered by filter capacitor 34 and filter resistor 36 and is available as voltage Vp to a load on line 14.
However, even in the absence of a load current, it is impossible to reach the theoretical voltage of three times the supply voltage because of the voltage drop across diodes 16, 18, and 20. The voltage on capacitor 22 is the voltage of the voltage source minus the voltage drop. Similarly, the voltage on capacitor 24 becomes the voltage on capacitor 22 minus the voltage drop across diode 18. And finally, the voltage available as the pumped voltage 14 is the voltage on capacitor 24 minus the voltage drop across diode 20. As the voltage of the voltage source 12 decreases the voltage drop across diodes 16, 18, and 20 becomes even more significant since the voltage drop becomes a larger percentage of the total voltage. It is therefore desirable to replace diodes 16, 18, and 20 with active components such as MOSFET transistors which have lower voltage drops than diodes, especially when the source voltage is below five volts.
Prior art FIG. 2 shows generally the same charge pump circuit as in FIG. 1, but with diode 16, 18, and 20 replaced with switches 40, 42, and 44, respectively, and with the addition of a timing control circuit 46. The switches 40, 42, and 44 are typically p-channel MOSFET's but can be n-channel MOSFET's, bipolar transistors, or the like. The timing control circuit 46 turns on switches 40, 42, and 44 at the same time that diodes 16, 18, and 20 would have turned on, respectively. Consequently, charge pump circuit 48 operates in an analogous manner as charge pump circuit 10 of FIG. 1. However, charge pump circuit 48 is advantageous since the voltage losses across switches 40, 42, and 44 is much lower than their counterpart diodes 16, 18, and 20.
More specifically, voltage source 12 is connected to the high voltage end of switch 40. The low voltage end of switch 40 is connected to the high voltage end of switch 42 and to the first end of capacitor 22. The low voltage end of switch 42 is connected to the high voltage end of switch 44 and the first plate of capacitor 24. A clock signal 26 is connected to the inputs of inverters 28 and 30. The output of inverter 28 is connected to the second plate of capacitor 22 and the output to inverter 30 is connected to the input to inverter 32. The output to inverter 32 is connected to the second plate of capacitor 24. The low voltage end of switch 44 is connected to filter capacitor 34, filter resistor 36, and the (pumped voltage) Vp 14.
In operation, the first plate of capacitor 22 is charged to the voltage source level through switch 40 when the clock signal 26 is high. When clock signal 26 goes low, the output of inverter 28 drives the second plate of capacitor 22 to the voltage source level. Consequently, the voltage on first plate of capacitor 22 is pumped to two times the supply voltage 12 minus the voltage drop across switch 40. At this time, the output to inverter 32 is low so that the second plate of capacitor 24 is at ground. If the voltage on capacitor 24 is less than the doubled voltage on capacitor 22, the charge on capacitor 22 will flow through switch 42 on to capacitor 24 charging it to the doubled voltage. On the next clock cycle, the output of inverter 32 goes high which drives the second plate of capacitor 24 to the voltage of the voltage source. Since the voltage on capacitor 24 was two times the voltage on the voltage source 12, the voltage on the first plate of capacitor 24 is now three times the voltage of the voltage source 12 (minus the voltage drops of the switches). The pumped voltage in FIG. 2 will be higher than the pumped voltage in FIG. 1 since the voltage drop across a switch is at least 10 times less than the voltage drop across a diode.
However, it has been observed that transistors can have problems when used as switches in the high voltage environment of a charge pump circuit. More specifically, some manufacturing process technologies, such as 1.2 micron BiCMOS process, are sensitive to "hot electrons" (large voltages) such as those present in the charge pump circuit. This means, for example, that it is not possible to have gate to drain voltages higher than 6 volts in a n-channel FET. Therefore, it is desirable to develop a circuit which has the low on resistance of a transistor and yet can handle the high voltages of a charge pump circuit.